1. Field of the Invention
The present invention relates to an AC/DC converting control circuit and more particularly, to a transformerless AC/DC converting control circuit.
2. Description of the Related Art
Transformers are widely used to convert alternating current into direct current. However, the traditional transformers are so large as to take space and too heavy for users to carry. Therefore, transformerless AC/DC converting apparatuses have been developed.
In general, the on/off current of the transformerless AC/DC converting apparatuses can be controlled by a circuit as shown in FIG. 1, in which the SCR circuit includes transistors Tr1, Tr2 and Tr3. When the input voltage begins to increase, base potential of the transistor Tr1 becomes higher than emitter potential thereof and the transistor Tr1 is switched on. During the period that the input voltage increases continuously, the base potential of the transistor Tr3 remains higher than the emitter potential until the base potential is reduced by the transistor Tr2, and then the transistor Tr3 is switched on and the transistor Tr1 is switched off (Takeahi, etc., 1995, Tokyo Metropolitan Industrial Technology pp. 165-168, xe2x80x9cLED Driver For Commercial Powerxe2x80x9d). Unfortunately, this design results in a serious oscillation problem, so that the output current cannot be well controlled by the transistor Tr1. FIG. 1A shows the unstable output voltage curve caused by the aforementioned oscillation, which further generates and accumulates undesired heat in the transistor Tr1.
Accordingly, the present invention provides a novel transformerless AC/DC converting control circuit to improve stability of the output current and thus prolong the life of the devices.
The object of the present invention is to provide a transformerless AC/DC converting control circuit, which has advantages of no oscillation, less heat accumulated, high efficiency, and longer life.
The transformerless AC/DC converting control circuit of the present invention with a load current flowing therethrough has a current input contact and a current output contact, which is suitable for illuminants, indicators, etc.
The control circuit primarily includes a current-switch circuit having a current input contact, a current output contact and a control contact, and being used to switch on/off and control the load current in accordance with received output signals from the duty-control circuit and the load-current limiting circuit; a duty-control circuit having a current input contact, a current output contact and a control contact, and being used to switch on or off the load current when a potential between the input and output contacts of the control circuit is lower or higher than predetermined values; and a load-current limiting circuit having a current input contact, a current output contact and a control contact, and being used to limit the load current in accordance with the current-switch circuit, when the load current is higher than a predetermined value.
Furthermore, the control contact of the duty-control circuit, the current input contact of the control circuit and the current input contact of the current-switch circuit are interconnected directly or indirectly by means of a conductive element. The current input contact of the load-current limiting circuit, the control contact of the current-switch circuit and the current input. contact of the duty-control circuit are interconnected directly or indirectly by means of a conductive element. The current output contact of the current-switch circuit and the control contact of the load-current limiting circuit are interconnected directly or indirectly by means of a conductive element. The current output contact of the duty-control circuit, the current output contact of the load-current limiting circuit and the current output contact of the control circuit are interconnected directly or indirectly by means of a conductive element.
In general, the current-switch circuit primarily includes a first transistor and a first resistor; the duty-control circuit primarily includes a second transistor, a second resistor and a third resistor; and the load-current limiting circuit primarily includes a third transistor and a fifth resistor.